1. Field of the Invention
The present invention relates to nuclear magnetic resonance devices and, more particular, to a low-field nuclear magnetic resonance device.
2. Description of the Related Art
When a nuclear spin of atoms constituting a substance is under an external magnetic field, the nuclear spin performs a precession around the external magnetic. In this case, if a specific radio frequency (RF) that is in proportion to the external magnetic field is externally applied, a resonance phenomenon is observed. This is called a nuclear magnetic resonance (NMR). Magnetic resonance imaging (MRI) is a technique for non-invasively imaging the inner part of an imaging object using the NMR. The MRI has been widely used as a medical diagnostics tool for imaging the inner part of human body.
Unlike conventional NMR/MRI systems operating under a strong magnetic field with the magnitude of several Tesla (T) to tens of T, ultra-low field (ULF) NMR/MRI systems are new-concept NMR/MRI systems operating under a magnetic field with the magnitude of several microTesla (μT) to tens of μT.
In the ULF-MRI system, a SQUID sensor is used to obtain a magnetic resonance signal under such a weak magnetic field, unlike conventional NMR/MRI systems. Since the SQUID sensor has very high sensitivity, an NMR signal under a magnetic field of several μT can be observed. The SQUID sensor measures superconducting screening current induced on a pickup coil. The measured signal does not have frequency dependence, therefore, unlike commercial NMR/MRI systems using an inductive coil. For this reason, the signal is not significantly reduced even under a low magnetic field. In addition, the SQUID sensor may be replaced by an optically-pumped atomic magnetometer which has the sensitivity comparable to that of the SQUID sensor.
However, the SQUID sensor is very sensitive to the fluctuation of an external magnetic field. Thus, a typical magnetic field of several to tens of Tesla cannot be used. Whereas, the magnetic field of several to tens of microTesla is not sufficient to magnetize a nuclear spin. Thus, the ULF-MRI system includes a prepolarization magnetic field and a measurement magnetic field. The magnitude of the prepolarization magnetic field is tens of milliTesla (mT) to hundreds of mT, and the magnitude of the measurement magnetic field is several μT to tens of μT.
The greater the magnitude of a prepolarization, the higher MRI imaging sensitivity. Therefore, it is advantageous to apply a large magnetic field. However, since a large magnetic field requires large current of tens to hundreds of amperes (A), there are many difficulties in implementing a driving circuit of a prepolarization coil to form the prepolarization magnetic field and cooling the prepolarization coil. Moreover, there is a spatial limitation in applying a large prepolarization magnetic field. The prepolarization coil may cause electrical interference with the SQUID sensor, and the development cost of the prepolarization coil is high. Accordingly, there is a need for a low-field nuclear magnetic resonance device that does not require the prepolarization coil.